A core pin is used for making a cast hole in a casting simultaneously with a casting process. Finishing a cast hole can reduce a machining allowance and the number of machining steps but also increase a material yield, as compared to machining a hole by means of a drill or the like.
However, because the core pin is inserted into a cavity and surrounded by high-temperature molten metal, a thermal load on the core pin would become great. As a measure for reducing the thermal load, a cooled (type) core pin is recommended which is cooled by a cooling medium, such as water (see, for example, Patent Literature 1). FIG. 18 hereof is a sectional view of an outer pin in the core pin disclosed in Patent Literature 1.
Referring to FIG. 18, the outer pin 100 has an annular groove 102 in its inner peripheral surface 101. Generally, such an annular groove 102 is formed by a boring method. Namely, a central hole is made in the material by means of a drill or the like. Then, a bore 105 having a blade section 104 at the distal end of a rod 103 is inserted through an inlet 106 and rotated relatively to shave off the material so as to form the annular groove 102.
It is essential that a maximum length L at the distal end of the bore 105 be smaller than a diameter of the inlet 106. The smaller the diameter of the inlet 106, the smaller becomes an outer diameter of the rod 103. As the outer diameter of the rod 103 becomes smaller, flexure is more likely to occur at the distal end of the rod 103. Therefore, with the boring method, a finishing accuracy of the annular groove 102 tends to be low. Additionally, it is difficult to provide the annular groove near the distal end 107 (remote from the inlet 106) of the outer pin 100.
However, depending on the core pin, it may sometimes be required that the annular groove 102 be also provided near the distal end 107. Thus, there has been a demand for a structure which allows the annular groove 102 to be provided at a desired position.